FCC technology has long been a predominant means of producing gasoline. In an FCC process, gasoline is formed as the result of cracking heavier (i.e. higher molecular weight), less valuable hydrocarbon feed stocks such as gas oil. Although FCC is a large and complex process involving many factors, a general outline of the technology is presented here in the context of its relation to the present invention.
The FCC process generally includes a reactor that is closely coupled with a regenerator, followed by downstream hydrocarbon product separation. Hydrocarbon feed 15 contacts catalyst in the reactor to crack the hydrocarbons down to smaller molecular weight products. During this process, the catalyst tends to accumulate coke thereon, which is burned off in the regenerator.
The heat of combustion in the regenerator typically produces a flue gas having an extremely high temperature. It is desirable to provide a power recovery device, such as expander turbine, to recover energy from these high-temperature flue gases. It is known, for example, to provide a turbine that can be coupled to an air blower to produce combustion air for the regenerator, a generator to produce electrical power.
The FCC process results in a continuous fluidization and circulation of large amounts of catalyst having an average particle diameter of about 50 to 100 microns, equivalent in size and appearance to very fine sand. For every ton of cracked product made, approximately 5 tons of catalyst are needed, hence the considerable circulation requirements. Coupled with this need for a large inventory and recycle of catalyst with small particle diameters is the ongoing challenge to prevent this catalyst from exiting the reactor/regenerator system into effluent streams.
The U.S. Environmental Protection Agency has limited emissions of catalyst from an FCC stack to 1.0 lb. of catalyst per lb. of coke regenerated. In particular situations, the emissions standard can be limited to 0.8 lbs. of catalyst per lb. of coke regenerated. It is desirable to reduce catalyst concentration in the flue gas to meet environmental regulatory emissions standards and also to provide a margin to ensure normal fluctuations in emissions will still be below environmental regulatory standards.
Additionally, catalyst particles are abrasive and thus are capable of damaging and eroding components located downstream of the regenerator, such as a turbine. If exposed to catalyst particles, blades of the turbine would erode and result in loss of power recovery efficiency. Moreover, even though catalyst fines; i.e., particles less than 10 μm in dimension, do not erode expander turbine blades as significantly, they still accumulate on the blades and casing. Blade accumulation can cause blade tip erosion and casing accumulation can increase the likelihood of the tip of the blade rubbing against the casing of the expander turbine which can result in high expander shaft vibration. Therefore, it is desirable to remove catalyst particles from the regenerator flue gas.
In order to remove solid catalyst particles, cyclone separators internal to both the reactor and regenerator have conventionally been implemented. Typically, the regenerator includes first and second (or primary and secondary) stage separators for the purpose of preventing catalyst contamination of the regenerator flue gas, which is essentially the resulting combustion product of catalyst coke in air. While normally sized catalyst particles are effectively removed in the internal regenerator cyclones, fines material (generally catalyst fragments smaller than about 50 microns resulting from attrition and erosion in the harsh, abrasive reactor/regenerator environment) is substantially more difficult to collect. As a result, the FCC flue gas will usually contain a particulate concentration in the range of about 100 to 500 mg/Nm3. This solids level can present difficulties related to the applicable legal emissions standards and are still high enough to risk damage to the power recovery expander turbine.
A further reduction in FCC flue gas fines loading is therefore often warranted, and may be obtained from a third stage separator (TSS). The term “third” in TSS typically presumes a first stage cyclone and a second stage cyclone are used for gas-solid separation upstream of the inlet to the TSS. These cyclones are typically located in the catalyst regeneration vessel. It is possible to provide more separator devices or fewer separator devices upstream of the TSS. Hence, as used herein, the term TSS does not require that exactly two separator devices are located upstream of the TSS vessel. The TSS induces centripetal acceleration to a particle-laden gas stream to force the higher-density solids to the outer edges of a spinning vortex. A conventional TSS vessel for an FCC flue gas effluent will normally contain a single stage of cyclone separators, including a deck on which a plurality of individual cyclones is installed within a single vessel. The deck includes upper and lower tube sheets affixing the upper and lower ends of the cyclones to distribute contaminated gas to the cyclone inlets and also to divide the region within the vessel into sections for collecting the separated gas and solid phases.
Examples of conventional TSS units having a single stage of cyclone separators are disclosed in U.S. Pat. Nos. 5,690,709; 6,673,133 and 6,797,026. Although such conventional TSS units have operated to remove a substantial proportion of particulates from the gas stream, it is desirable to provide a TSS that yields an increased reduction in particulate fines.